Human embryonic stem cells (hESC), first isolated from the inner cell mass of the blastocyst, possess dual capabilities of extensive self renewal and multilineage differentiation. Their proliferative capacity, together with the ability to become specialized, makes hESC suitable for use in regenerative medicine and drug discovery (1,2). However, cell surface markers routinely used for characterizing hESC are not unique as they have been used to characterize human embryonal carcinoma (EC) cells (3,4). Therefore, it is necessary to identify novel antigens, specific to hESC, for efficient evaluation of pluripotency and different stages of development during differentiation.
To achieve this, we have previously reported the generation of monoclonal antibodies (mAb) that bind specifically to undifferentiated hESC (5) and in this study, we have identified the common antigen target for two of the mAbs (mAb 375 and mAb 529) as Prohibitin (PHB).
PHB is a highly conserved protein in eukaryotic cells and is present in multiple cellular compartments such as the mitochondria (6,7), nucleus (8-10) and plasma membrane (11-13). In addition to its role as chaperone proteins in the mitochondria (6,7), PHB also modulates cell proliferation in cancer cells, when it localizes to the nucleus (8,10,14). In a detailed study on breast cancer cells by Wang et al, PHB has been reported to interact with retinoblastoma protein (Rb) and regulates E2F transcriptional activity and this regulation correlates with its growth-suppressive activity (10,15,16). Recent studies have also highlighted PHB as a surface vascular marker of adipose tissue and it can be used as an internalization receptor that may be used for targeted delivery of therapeutic compounds (11). PHB has also been shown to be indispensable for activation of ERK pathway, where direct interaction with PHB is required for C-Raf activation (13). Since Ras mutations are commonly found in tumours (17), PHB could potentially be a target for tumour therapy.
The use of embryonic stem cells in medicine is limited due to the significant ethical concerns associated with the use of embryos. Recently, the Yamanaka Lab (Takahashi et al (2007) Induction of Pluripotent Stem Cells from Adult Human Fibroblasts by Defined Factors. Cell 131(5):861-72) and Thomson Lab (Yu et al (2007) Induced Pluripotent Stem Cell Lines Derived from Human Somatic Cells. Science 318(5858):1917-20) demonstrated that human fibroblasts can be reprogrammed by the transient overexpression of a small number of genes into induced pluripotent stem cells (IPSCs) which functionally and phenotypically resemble embryonic stem cells (ESCs). Thus, pluripotent stem cells can be obtained without the need for the destruction of embryos.
Although undifferentiated stem cells may be used in cell therapy, it is considered to be beneficial to use cells which have started to differentiate, or are differentiated. Methods of encouraging stem cells to differentiate into particular cell lineages are well known in the art. Once the differentiation process has started or proceeded, it is beneficial to remove or destroy undifferentiated hESCs in a sample which may otherwise form undesirable teratomas. Teratomas typically contain a mixture of differentiated or partly differentiated cell types. Despite the potential of IPSC therapy, the problem of teratoma formation by residual IPSC after differentiation remains and needs to be addressed.